Earthquakes? Blame the Kapiti creep

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Damaging earthquakes that have shaken central New Zealand strongly in the past year appear linked to a puzzling, creeping phenomenon under the lower North Island.

Scientists believe "silent earthquakes" along and off the Kapiti coast have transferred ground stress elsewhere in the region, possibly triggering recent quake swarms in Marlborough, the Wairarapa and Hawke's Bay.

Silent quakes - also known as slow-slip events (SSE) - cannot be felt by people and can only be detected by using global positioning system (GPS) instruments to measure ground movements of just a few millimetres.

They occur where one crustal plate slides below another - in this case the Pacific plate is being forced under the Australian plate.

Dr Laura Wallace, of the University of Texas, and Dr Bill Fry, a GNS Science seismologist, think these slow quakes are helping relieve tectonic tensions built up over hundreds of years between the two plates.

The latest swarm of strong quakes likely to have been affected by the Kapiti creep was centred around Waipukurau in central Hawke's Bay on Monday , including a magnitude 5.4 shake.

In January, a magnitude 6.3 quake struck further southwest between Eketahuna and Castlepoint in the Wairarapa, and in July and August last year there were significant 6.5 and 6.6-magnitude shocks under Cook Strait and close to the Marlborough town of Seddon.

Fry said the large Kapiti silent quake that had been under way since early 2013 had released about the same amount of energy from the crust as a magnitude 7.1 quake - equal to the September 4, 2010, Canterbury event.

That slow-quake energy had changed the stress on faults in the surrounding area.

The effect of that, along with a small contribution from stress changes from the Seddon-Cook Strait quakes, had increased tension on faults near Eketahuna.

That had also affected stress on faults around Waipukurau, meaning they were then "pushed towards failure".

"It seems very likely the Kapiti SSE has had a major influence on the earthquakes that have been experienced in central New Zealand for the last year."

Unfortunately, they could not calculate the risk of further earthquakes elsewhere, not knowing what stress was already loaded on those faults, Fry said.

"All the stress calculations do is give us an estimate of how much closer the fault is to failure. We don't know its starting stress, so we can't quantify the increased likelihood of an event using known physics."

Another quake sequence in the Wairarapa in 1990, with magnitude 5.9 and 6.2 quakes, was probably also linked to a slow quake, he said. Previous Kapiti silent quakes in 2003 and 2008 had also produced higher levels of quake activity in the lower North Island.

Wallace said the slow-slip's 7.1-magnitude made it the largest silent quake seen in New Zealand since scientists were able to use GPS to detect such events.

Researchers are looking at drilling into the silent-quake zone in the Hikurangi subduction zone east of the North Island to find out more about them and their links to massive 8 or 9 magnitude megathrust earthquakes, which generate deadly tsunamis like the Boxing Day 2004 magnitude 9.2 Indonesian quake and the March 2011 9.0 Japanese one.